1. Field of the Invention
The present invention related to a mounting structure and a mounting method of an electronic part such as a semiconductor element, a chip type capacitance or a resistor, which is electrically connected to a mounting substrate via electrodes.
2. Related Arts
In one conventional mounting structure for an electric circuit device such as a HIC (Hybrid Integrated Circuit), when surface mounting electronic part is mounted on a substrate, a conductive adhesive such as Ag (silver) paste is used to mount them for the purpose of Pb(lead)-free or fleon-free (elimination of a washing process for washing flux remains).
FIG. 15 shows the above-mentioned conventional mounting structure. In this figure, a laminated ceramic capacitor 42 for surface mounting, which has a pair of electrodes 43, is mounted on a mounting substrate 41 made by an insulating material such as ceramic or resin. This mounting structure is obtained by following process. That is, the laminated ceramic capacitor 42 is mounted on the mounting substrate 41. The Ag paste 44 is transcribed on the pair of electrodes 43 of the mounting substrate 41 by a screen printing. The laminated ceramic capacitor 42 is mounted on the Ag paste 44 with a predetermined load and time condition. Then Ag paste is hardened.
According to a thermal cycle test (from -40.degree. C. to 150.degree. C., 1000 cycles) on the above-described conventional mounting structure, it is found that cracks are generated in some interfaces, and that junctions of the interfaces may be deteriorated by the cracks. Here, the interfaces include the one between electrodes 42a of the laminated ceramic capacitor 42 and the Ag paste 44, and between the electrodes 43 of the mounting substrate 41 and the Ag paste 44. These cracks may occur in the following manner.
In a mounting structure using a reflow soldering, flux components remove dirt such as oxide compounds or organic matters on the electrodes of the electronic part and the electrodes of the mounting substrate, and the solder and electrode materials are metallically bonded each other due to a reflow heating. Hence, a dependency of a bonded state against the electrode materials is rather small.
On the other hand, in the mounting structure using Ag paste 44, Ag fillers, which exist in the Ag paste 44 independently, are connected in chains each other and are connected to each electrode 42a, 43 in proportion to hardening and contraction of a binder resin in the Ag paste 44.
Therefore, the bonded state is likely to be affected by the electrode material (specifically surface material), and an adhesive strength between each of the electrodes 42a, 43 and the Ag paste becomes small compared to that of soldering.
According to investigation to check where the cracks are generated in the Ag paste 44 in the thermal cycle test, it is found that the cracks are generated mainly at peripheral portion of bonded portions between Ag paste 44 and the electrodes 42a, 43. That is, the cracks are generated at bonded portions where the adhesive strength becomes small due to heating contractions of Ag paste 44 under high temperature.
In another conventional mounting structure, an electronic part having a narrow pitch land (electrode) such as a semiconductor element is mounted on a mounting substrate by a flip chip mounting by using a solder. This mounting structure is obtained by following steps shown in FIGS. 16A-16D.
As shown in FIG. 16A, a mounting substrate 52 having electrodes (electrodes or lands) 51 is provided. As shown in FIG. 16B, solder pastes 53 are printed on the mounting substrate 52. Here, a diameter of each solder pastes 53 is substantially equal to that of land 51 to obtain desired bonded lifetime of the solder. As shown in FIG. 16C, a semiconductor element having solder bumps 54 is aligned and mounted on the mounting substrate. The solder bumps 54 and the solder pastes 53 are bonded by melting each other by a reflow process. Then, the semiconductor element 55 is electrically connected to the mounting substrate 52, as shown in FIG. 16D.
In this conventional mounting structure, one of a pattern tolerance of the lands 51, a printing displacement of the solder paste 53, and a mounting displacement of the semiconductor element relative to the mounting substrate may occur. Hence, the lands 51 and the solder paste 55 may be displaced each other. However, the solder paste 53 and the solder bump 54 are melted, and spread on the lands 53, then the melted solder will return to the lands 51. The solder pastes 53 need to contact at more than half area with the lands 51 to have the melted solder return to the lands 51 completely. On the other hand, when the solder pastes 53 contact the lands 51 at less than half area, the melted solder may not return to the lands 51 completely. In such a case, the melted solder may remain on the mounting substrate as a solder ball, or be merged with the adjacent solder at the adjacent land 51, then the adjacent lands 51 may be short-circuit by the melted solder as shown in FIG. 17.
Consequently, reliability of electrical connection of the semiconductor element or the electronic part may decrease due to the decreasing of an amount of the solder of lands 51 or short-circuit between adjacent lands 51.
Recently, a land pitch of the semiconductor element 55 is desired to be less than 300 .mu.m for the purpose of fining. When the ceramic laminated substrate is employed as the mounting substrate 52, the pattern accuracy may be decreased due to non-uniformity of baking contraction, and a size tolerance of around 1% will be generated. Therefore, the following accuracy is severely required to satisfy this desire. Here, the accuracy includes a pattern accuracy of the land 51 provided on the mounting substrate 52, a printing alignment of the solder pastes 53, accuracy of weight of each solder paste, and a mounting accuracy of the semiconductor element 55 to the mounting substrate 52.
However, when the semiconductor element has a size of 10 mm.times.10 mm, at a peripheral portion of the semiconductor element 55, the pattern tolerance may be 0.07 mm, the printing displacement may be 0.05 mm, and the mounting displacement may be 0.03 mm. In this case, the displacement between the solder paste 53 and the solder bump 54 with respect to the land 51 is estimated to be around 0.13 mm by calculating root mean square of each displacement. When the land pitch is set to be less than this estimated value, adjacent solders may be merged to short-circuit.
However, since the size of land 51 is determined based on the bonded lifetime or predetermined displacements, the land pitch can not be set to be less than 0.28 mm when the land size is set to 0.15 mm.